Aluminum alloy sheet excellent in high-speed superplastic formability and process of forming the same

ABSTRACT

An alloy containing 3.0 to 8.0 wt. % of Mg, 0.001 to 0.1 wt. % of Ti, and amounts of Fe and Si (as impurities) as small as 0.06 wt. % or less, the balance being Al and unavoidable impurities, wherein the number per square millimeter of grains of an Al—Fe—Si compound having a diameter of 1 μm or more is 2000 or less, the mean crystal grain diameter is 25 to 200 μm and the elongation is 350% or more as worked at 350 to 550° C. and a strain rate of 10 −2  to 10 0 /s. This alloy may further contain a small amount of Cu, Mn and/or Cr and is formed at 350 to 550° C. and a strain rate of 10 −3  to 10 0 /s. An aluminum alloy sheet with excellent high strain rate superplastic formability and therefore capable of being formed at high temperature and high speed is obtained and the use of this sheet shortens the forming time to improve productivity.

FIELD OF THE INVENTION

[0001] This invention relates to an aluminum alloy sheet which hasexcellent high-speed superplastic formability, and more specifically, toan Al—Mg alloy sheet which enables superplastic forming at high strainrate of 10⁻² to 10⁰/s; and also to a process for forming the same.

BACKGROUND OF THE INVENTION

[0002] Based on Al—Mg alloy systems, by using a technique of regulatingrecrystallization to obtain finer crystal grains, superplastic alloyshaving an elongation of several hundred percent in high temperatureranges such as that between 500 to 550° C. have been developed and arebeing used in various applications. However, conventional Al—Mgsuperplastic alloys demonstrate the best elongation at a forming speed(i.e. strain rate) between 10⁻⁴ to 10⁻³/s, at which it takes 30 to 100minutes, for example, to form an ordinary utensil. This is anunacceptably low productivity for a commercial manufacturing process.Superplastic alloys that can be formed at a much higher forming speedare therefore required.

[0003] For example, an aluminum alloy sheet containing 2.0 to 6.0% ofMg, 0.0001 to 0.01% of Be, and 0.001 to 0.15% of Ti, with Fe and Si asimpurities being controlled each at 0.2% or less and the largest graindiameter of impurity-based intermetallic compounds limited to 10 μm orless is proposed in Japanese Patent Application Laid-Open No.72030/1992. While such a product does show an elongation of 350% or moreat a strain rate of 10⁻³/s under a high-temperature deformationcondition of 400° C., the elongation decreases as the forming speedincreases and becomes insufficient at strain rates of 10⁻²/s or higher.

[0004] Another aluminum alloy sheet, proposed in Japanese PatentApplication Laid-Open No. 318145/1992, contains 2 to 5% of Mg, 0.04 to0.10% of Cu, as well as optional small quantities of certain transitionelements, Cr, Zr, or Mn; with Si and Fe as impurities being controlledat 0.1% or less, and at 0.15% or less, respectively; while controllingthe crystal grain diameter at 20 μm or less and maintaining the graindiameter and the cubic ratio of transition metal-based intermetalliccompounds within certain specific ranges. Such an alloy sheet also has alimited application range of strain rates in the order of 10⁻⁴/s, and isnot suitable for high strain rate superplastic forming at a higherstrain rate.

SUMMARY OF THE INVENTION

[0005] The present invention has been achieved as a result of diverseexamination and exhaustive experiments concerning the relationships ofsuperplastic formability with various alloy constituents and theirquantitative combinations, in addition to those with impurity contentand their distribution, as well as with crystal grain diameters ofimpurity-based intermetallic compounds, made in an attempt to overcomethe aforementioned shortcomings of the Al—Mg superplastic aluminumalloy. In particular, the object of the present invention is to provide,by identifying a particular distribution and crystal grain diameterrange for Al—Fe—Si compounds to be controlled based on restriction of Feand Si as impurities, an aluminum alloy sheet that has excellent highstrain rate superplastic formability with sufficient elongation in aforming process with a high forming speed such as at a strain rateranging from 10⁻² to 10⁰/s, as well as to provide a processing methodfor forming such an aluminum alloy sheet.

[0006] To achieve this object, the aluminum alloy sheet with excellenthigh strain rate superplastic formability in the present inventioncomprises 3.0 to 8.0% Mg, 0.001 to 0.1% Ti, small amounts of Fe and Si(as impurities), each 0.06% or less, the balance being Al andunavoidable impurities, wherein the number per square millimeter ofgrains of an Al—Fe—Si compound existing in the matrix structure of saidalloy and having a diameter of 1 μm or above is 2000 or less, the meancrystal grain diameter is 25 to 200 μm and the elongation is 350% ormore as worked at 350 to 550° C. and a strain rate of 10⁻² to 10⁰/s, allof the foregoing constituting the basic features of the invention.

[0007] As the second and the third features constituting the invention,this alloy may further comprise 0.05 to 0.50% of Cu in addition to Mgand Ti as described above; or may comprise either one or both of Mn orCr not exceeding 0.10% each in addition to Mg and Ti as described above,or alternatively, in addition to Mg, Ti, and Cu as described herein.

[0008] The processing for fabricating the aluminum alloy sheet withexcellent high-speed superplastic formability in the present inventionis characterized by working an aluminum alloy sheet prepared inaccordance with the invention at 350 to 550° C. and a strain rate of10⁻³ to 10⁰/s.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0009] Referring to the significance of the alloy constituents describedin this invention and the basis of their stated limits, first of all, Mgacts to recrystallize the alloy during the high-temperature deformation.The preferred content range is between 3.0 and 8.0%, below which theeffect on promoting the recrystallization is insufficient while acontent in excess of 8.0% acts to reduce hot workability of thematerial. Cu on the other hand acts to improve the superplasticelongation of the Al—Mg alloy system. The preferred content range isbetween 0.05 to 0.50%, where a content below 0.05% fails to givesufficient elongation while a content in excess of 0.50% acts to reducethe hot workability.

[0010] Ti acts to turn the ingot crystals into finer grains and toprovide the alloy with a better superplastic formability. The preferredcontent range is between 0.001 to 0.1%, where a content below 0.001%will fail to give the expected effect and a content in excess of 0.1%will yield coarse compounds that hinder workability as well asductility. Further, Mn and Cr act to make recrystallized grains finer inthe alloy recrystallization process that occurs during high-temperaturedeformation. The preferred content range is below 0.10% for each, wherea content in excess of 0.10% will act to increase a constituent particlewhose grain diameter is 1 μm or above to decrease the superplasticformability of the alloy.

[0011] In the present invention, it is essential to limit Fe and Si asimpurities each at 0.06% or less. These impurities form an Al—Fe—Sicompound that is insoluble and prone to precipitate along the grainboundary, increasing cavities and thereby impairing the superplasticelongation. Preferably, the Fe and Si should each be controlled at 0.05%or less. It is also noted here that up to 50 ppm of Be may be added toprevent oxidation of the molten metal, just as in the case of ordinaryAl—Mg alloys.

[0012] Referring further to the alloy structure of the presentinvention, since the Al—Fe—Si compound present in the alloy matrix givesrise to the above mentioned problem, it is better to allow as little ofsuch a compound as possible, and, in particular, the limit in terms ofnumber per square millimeter of an Al—Fe—Si compound having a graindiameter of 1 μm or more should be 2000 or less, since particles inexcess of 2000 per square millimeter will increase cavities and therebyimpair the superplastic elongation.

[0013] It is essential to regulate the original mean crystal graindiameter of the aluminum alloy sheet within a range of 25 to 200 μm. Ifthe original mean crystal grain diameter is below 25 μm, the originalcrystal grains will be recreated when recrystallization occurs duringhigh temperature deformation, making it difficult to obtain arecrystallized structure with clean crystal grains as a result of arecrystallization process to obliterate the grain boundary withprecipitation of the aforementioned insoluble compounds. If the originalmean crystal grain diameter exceeds 200 μm, the shearing deformationwithin the crystal grains becomes more prominent with increasingdeformation rate, causing the crystal grains to rupture more easily,thus suppressing the superplastic elongation.

[0014] It is preferable to carry out a forming process for the aluminumalloy sheet of the present invention at a temperature between 350 to550° C. At a temperature below 350° C., Al—Mg or Al—Mg—Cu compounds areprone to precipitate along the grain boundary to lower the elongation.Conversely, at a forming temperature exceeding 550° C., the crystalgrains tend to become coarse, adversely affecting the elongation. Thepreferred range of the strain rate during the forming process is between10⁻³ to 10⁰/s, where a rate below 10⁻³/s will cause the crystal grainsto become coarser, reducing elongation, while a strain rate exceeding10⁰/s creates a shearing deformation within the crystal grains causingcracks, or forms precipitation along the grain boundary, reducingelongation.

[0015] As a procedure for preparing the aluminum alloy sheet in thepresent invention, an aluminum alloy material with the above mentionedcomposition is melted, cast, and homogenized according to a conventionalmethod. It is preferable to carry out the homogenizing process at atemperature between 450 to 550° C. At temperatures below 450° C., Mg orCu that are formed along the grain boundary or the cell boundary of theingot by segregation will not be fully dissolved and may contribute tocracks in a subsequent hot rolling step. Conversely, at temperaturesexceeding 550° C., the Al—Mg or Al—Mg—Cu crystallization products willcause a eutectic fusion thereby giving rise to cracks during the hotrolling process.

[0016] After the homogenization process, the ingot is hot-rolled toobtain a structure suitable as a forming material. While the requiredstarting temperature for hot rolling is between 250 to 500° C., it ispreferable to start just under 400° C. If the hot rolling process isstarted at a temperature below 250° C., the deformation resistance istoo high, making proper rolling difficult. If the rolling temperature istoo high, this could alter the distribution form of the precipitation,thereby making it difficult to obtain the required crystal grainstructure as well as proper distribution of precipitated compounds.

[0017] Following the hot rolling process, a cold rolling is provided. Inaddition, an intermediate annealing may be provided as necessary. Thefinal annealing of the cold rolled stock should be provided at atemperature between 350 to 550° C. If the annealing is performed at atemperature below 350° C., the isotropy created during the cold rollingprocess may not completely disappear; if higher than 550° C., a localmelting may occur at the recrystallization boundary. As such, it ispreferred to conduct the final annealing in a rapid annealing processsuch as continuous annealing.

[0018] In the present invention, by restricting content of Fe and Si asimpurities in an Al—Mg alloy system and adjusting the combination ofmanufacturing conditions to the combination of the alloy constituents asdescribed above, the Al—Fe—Si compounds present in the matrix arecontrolled within certain specific distribution while maintaining thecrystal grain diameter within a certain specific range, resulting insuch alloy structure and characteristics to produce cleaner grainboundaries with less compounds formed along these bounderies to suppresscavity formation. Recrystallized grains having an average diameter of 20μm or less are formed during a high-temperature deformation, therebyachieving an excellent elongation of 350%, or more preferably, 380% orgreater, in high speed forming at a strain rate of 10⁻² to 10⁰/s in atemperature range of 350 to 550° C.

EXAMPLES

[0019] The following describes examples of practical applications andcomparative experiments pertaining to the present invention.

Example 1, Comparative Example 1

[0020] Al—Mg based aluminum alloys having compositions as listed inTable-1 below were melted and cast into ingots via a DC casting method.The resultant ingots were homogenized at 530° C. for 10 hours to athickness of 30 mm, and then hot rolled at 390° C. to a thickness of 4mm. The sheets were subsequently cold rolled to a thickness of 2 mm andthen rapidly annealed by heating rapidly to 480° C. and holding at thistemperature for 5 minutes. Specimens prepared from the test materialsproduced in the above process were evaluated by a tensile test at astrain rate of 10⁻²/s at 480° C. Table 1 lists the average crystal graindiameter for each specimen(as measured at the sheet surface), the numberper square millimeter of grains of the AL—Fe—Si compound having adiameter of 1 μm or above, and the elongation measurement results. Notehere that the grain count of the compound was made using imageprocessing. TABLE 1 Average Al—Fe—Si Compound, Composition (wt %)Crystal Grain ø 1 μm or larger Elongation Material Mg Cu Ti Fe SiDiameter (μm) (Nos./mm²) (%) 1 5.4 — 0.02 0.05 0.05 60 1220 455 2 7.3 —0.02 0.04 0.04 55 1360 420 3 5.4 — 0.01 0.01 0.01 130   250 480 4 5.00.45 0.02 0.06 0.06 45 1460 520 5 5.4 0.3  0.02 0.04 0.05 55 1270 560 66.5 0.6  0.05 0.07 0.06 — — — 7 8.5 — 0.05 0.03 0.04 — — — 8 5.2 — 0.040.12 0.15 15 3550 220 9 2.8 — 0.02 0.05 0.05 45 1180 280

[0021] As shown in Table 1, all of Materials No. 1 through 5 whichcomply with the present invention demonstrated elongation exceeding400%. On the other hand, both Material No. 6 with an excessive Cucontent, and Material No. 7 with excessive Mg developed cracks duringthe hot rolling process and failed to produce specimens. Further,Material No. 8 shows inferior elongation due to the excessive amount ofimpurities Fe and Si and the resultant number of large compound grains.Finally, Material No. 9, containing insufficient amount of Mg, alsoshows poor elongation due to lack of recrystallization during thestretching deformation.

Example 2, Comparative Example 2

[0022] Al—Mg based aluminum alloys having compositions as listed inTable 2 were melted and cast into ingots in the same manner as inExamples 1, and made into 2-mm thick test materials using the sameprocess as in Examples 1. Specimens were then evaluated in the sametensile test under the same conditions. Table 2 lists the averagecrystal grain diameter, the number per square millimeter of grains ofthe AL—Fe—Si compound having a diameter of 1 μm or above, and theelongation measurement results. TABLE 2 Average Al—Fe—Si Compound,Composition (wt %) Crystal Grain ø 1 μm or larger Elongation Material MgCu Ti Mn Cr Fe Si Diameter (μm) (Nos./mm²) (%) 10 5.5 — 0.02 0.04 0.040.05 0.05 35 1330 410 11 5.4 0.3  0.02 0.05 0.01 0.04 0.04 25 1280 39012 5.5 — 0.01 0.01 0.05 0.01 0.01 40  550 500 13 5.3 — 0.02 0.12 0.010.06 0.06 15 2730 280 14 5.4 — 0.03 0.15 0.10 0.04 0.05 11 3570 210 155.5 0.25 0.02 0.08 0.12 0.07 0.06 11 3240 240

[0023] As shown in Table 2, all the Materials No. 10 through 12 whichcomply with the present invention demonstrated elongation exceeding380%. However, both Materials No. 13 and No. 14, with their excessive Mncontent, as well as Material No. 15 with its excessive Cr content allshowed inferior elongation due to excessive distribution of the Al—Fe—Sicompound grains equal to or larger than 1 μm in diameter.

Examples 3, Comparative Example 3

[0024] An aluminum alloy having the same composition as Material No. 5in Examples 1 was melted and cast in the same manner as in the Examples1, and the resultant ingot was homogenized at 520° C. for 8 hours to athickness of 30 mm, then hot-rolled starting at 390° C. to a thicknessof 4 mm. The sheet was subsequently cold-rolled to a thickness of 2 mmand then rapidly annealed by heating rapidly to 480° C. and holdingthere for 5 minutes. Specimens prepared from the test material producedin the above process were evaluated in a tensile test with varyingstrain rates and forming temperatures as indicated in Table 3. Theelongation measurement results are as shown in Table 3. For guidance,the average crystal grain diameter (as measured at the sheet surface)for all of these specimens was in the range of 50 to 60 μm, and thenumber per square millimeter of grains of the AL—Fe—Si compound having adiameter of 1 μm or above, likewise, was below 2000. TABLE 3 TensileTest Elongation Material Temperature (° C.) Strain Rate(/s) (%) 16 45010⁻² 480 17 180 10⁻² 540 18 480 10⁻¹ 410 19 520 10⁻² 450 20 350 5 × 10⁻³380 21 580 10⁻²  30 22 480 5 × 10⁻⁴ 280 23 480 2 × 10⁰   80

[0025] As shown in Table 3, all the Materials No. 16 through 20 whichcomply with the present invention demonstrated elongation equal to orgreater than 380%. However, the Maerial No. 21 showed a diminishedelongation result due to its high tensile test temperature whichresulted in coarse crystal grains. The Material No. 22, on the otherhand, showed poor elongation due to coarse crystal grains formed duringdeformation because of too small a strain rate. Finally, the MaterialNo. 23 with too high a strain rate employed also showed inferiorelongation.

INDUSTRIAL APPLICATION

[0026] As described in the foregoing, the present invention prvides anAl—Mg aluminum alloy sheet having excellent superplastic elongation inhigh speed forming such as at high strain rate of 10⁻² to 10⁰/s at ahigh temperature, and a superplastic forming process using this aluminumalloy sheet shortens the forming time to improve productivity.

What is claimed is:
 1. An aluminum alloy sheet with excellent high-speed superplastic formability comprising an alloy containing 3.0 to 8.0% (% by weight, hereinafter the same) of Mg, 0.001 to 0.1% of Ti, controlled amounts of Fe and Si (as impurities) each at 0.06% or less, and the balance being Al and unavoidable impurities, wherein the number per square millimeter of grains of an Al—Fe—Si compound existing in the matrix structure of said alloy and having a diameter of 1 μm or more is 2000 or less, the mean crystal grain diameter is 25 to 200 μm and the elongation is 350% or more as worked at a temperature range of 350 to 550° C. and a strain rate of 10⁻² to 10⁰/s.
 2. The aluminum alloy sheet with excellent high-speed superplastic formability according to claim 1 , wherein said alloy further comprises 0.05 to 0.50% of Cu.
 3. The aluminum alloy sheet with excellent high-speed superplastic formability according to claim 1 or claim 2 , wherein said alloy further comprises either one or both of not more than 0.10% of Mn or not more than 0.1% of Cr.
 4. A method for forming the aluminum alloy sheet with excellent high-speed superplastic formability according to claim 1 , claim 2 , or claim 3 , wherein the aluminum alloy sheet is formed at a strain rate of 10⁻³ to 10⁰/s and at a temperature between 350 to 550° C. 